1. Field of the Invention
The present invention relates to a control apparatus for brushless dc motors and more particularly to a control apparatus for a brushless dc motor which is designed so that an electric current flowing through each of the armature coils of the brushless dc motor is chopper-controlled. "A brushless dc motor" is hereinafter referred to simply as "a brushless motor".
2. Description of the Related Art
A control apparatus for a brushless motor is known in the art in which respective series circuits of armature coils 2a, 2b and 2c for respective phases of the brushless motor and power transistors 3a, 3b and 3c are connected across a dc power source 1, as shown in FIG. 5.
Further, series circuits of control transistors 4a, 4b and 4c for respectively controlling the power transistors 3a, 3b and 3c and resistors 5a, 5b and 5c are connected between a positive terminal of the dc power source 1 and respective bases of the power transistors 3a, 3b and 3c.
In FIG. 5, reference numeral 6 designates a comparator, which has its negative signal input terminal 6a connected to the junction point between the positive terminal of the dc power source 1 and one end of the armature coil 2c and has its positive signal input terminal 6b connected to the other end of the armature coil 2c. As power supply to the comparator 6, its positive power input terminal 6c is connected to a constant voltage circuit 7 and its negative power input terminal 6d is connected to the negative terminal of the dc power source 1. Thus, the output of the constant voltage circuit 7 is connected, as a single positive voltage supply, only to the positive power input terminal 6c of the comparator 6 to thereby reduce the cost. It should be noted that, while FIG. 5 shows that the signal input terminals of the comparator 6 are connected only across the armature coil 2c, a similar comparator (not shown) is connected across each of the other armature coils 2a and 2b.
Reference numeral 8 designates a base drive circuit which receives the outputs of the comparators 6 and drives the control transistors 4a, 4b and 4c, respectively.
Reference numeral 9 designates a two-pole rotor composed of a permanent magnet having S and N poles.
Here, the method of driving the rotor 9 is known from Japanese Unexamined Patent Publications No. 55-160980 and No. 57-173385, for example. In the method disclosed therein, a time point, where positive-to-negative or negative-to-positive transistion occurs in an induced voltage in each of the armature coils 2a, 2b and 2c, is detected by the comparator 6 and the detection outputs therefrom are applied to the base drive circuit 8. In response to the detection outputs from the comparators 6, the base drive circuit 8 sequentially drives the control transistors 4a, 4b and 4c so that the control transistors 4a, 4b and 4c respectively switch electric currents flowing through the armature coils 2a, 2b and 2c, thereby driving the rotor 9 to rotate. Thus, the rotational position of the rotor 9 is detected by using the induced voltages in the armature coils 2a, 2b and 2c, and hence there is no need to use Hall generators or the like in order to detect the rotational position of the rotor 9.
In addition, a chopper circuit 10 is connected between the dc power source 1 and the respective phase armature coils 2a, 2b and 2c. The chopper circuit 10 comprises a power transistor 11, a free-wheeling diode 12 connected in parallel with the respective series connection circuits composed of the armature coils 2a, 2b and 2c and the power transistors 3a, 3b and 3c, and a chopper control circuit 13 for driving the base of the power transistor 11 at a given duty cycle. With this construction, the voltages applied to the respective phase armature coils 2a, 2b and 2c can be varied by the PWM control performed by the chopper circuit 10, whereby a variable speed operation of the brushless motor can be effected through the speed range from a lower speed to a high speed.
With this conventional control apparatus, however, mere provision of the chopper circuit 10 for effecting a variable speed operation has disadvantages such as described hereunder.
(a) As the PWM control is performed, a voltage across the free-wheeling diode 12 which provides a reference level for the induced voltages in the armature coils 2a, 2b and 2c varies in the positive and negative directions. In other words, when the power transistor 11 is turned on, the positive power supply voltage E.sub.B of the dc power source 1 is applied across the free-wheeling diode 12, but when the power transistor 11 is turned off, due to the winding inductance of the armature coil 2a, an electric current is caused to flow through the free-wheeling diode 12. Consequently, a negative voltage corresponding to a voltage drop caused by the electric current flowing through the free-wheeling diode 12 in the forward direction appears on the side of the cathode terminal of the free-wheeling diode 12 with respect to the GND terminal of the dc power source 1.
Therefore, the voltage waveform appearing between the point A in FIG. 5 and the GND (ground), which provides a reference level for the discrimination of the induced voltages in the armature coils 2a, 2b and 2c, is affected by the switching waveform which functions as an in-phase noise signal, as described above.
(b) Since the potential at the point A in FIG. 5 becomes negative when the free-wheeling diode 12 is turned on, as mentioned above, the mere connection of the single positive voltage supply, as a power supply input, to the positive power input terminal 6c of the comparator 6 does not allow the comparator 6 to perform an accurate operation due to the above-mentioned negative potential, and, as a result, it becomes impossible to determine the rotational position of the rotor 9.
(c) As mentioned above, it is not possible for the connectional apparatus shown in FIG. 5 to effect the desired PWM control of the voltages applied the respective armature coils of the brushless motor without needing the use of separate sensors for detecting the rotational position of the rotor 9.